Band-pass filters are used in an ever-widening variety of electronic products, especially wireless products. An ideal band-pass filter provides little or no attenuation of signals at frequencies in the pass band, has a flat frequency response in the pass band, and provides a large attenuation of signals at frequencies outside the pass-band. Additionally, in an ideal band-pass filter, the slope of the transition between the pass band and the stop band is steep.
Band-pass filters that incorporate one or more film bulk acoustic resonators (FBARs) have characteristics that approximate the above-described ideal characteristics yet are small in size and are relatively inexpensive. For example, modern cellular telephones incorporate a duplexer composed of two band-pass filters. Each of the band-pass filters includes a ladder circuit in which each element of the ladder circuit is an FBAR. A duplexer incorporating FBARs is disclosed by Bradley et al. in U. S. Pat. No. 6,262,637 entitled Duplexer Incorporating Thin-film Bulk Acoustic Resonators (FBARs), assigned to the assignee of this disclosure and incorporated by reference. Such duplexer is composed of a transmitter band-pass filter connected between the output of the transmitter and the antenna and a receiver band-pass filter and a 90° phase-shifter connected in series between the antenna and the input of the receiver. The center frequencies of the pass-bands of the transmitter band-pass filter and the receiver band-pass filter are offset from one another. Band-pass filters incorporating ladder filters based on FBARs are also used in other applications.
FIG. 1 shows an exemplary embodiment of an FBAR-based band-pass filter 10 suitable for use as the transmitter band-pass filter of a duplexer. The transmitter band-pass filter is composed of series FBARs 12 and shunt FBARs 14 connected in a ladder circuit. Series FBARs 12 have a higher resonant frequency than shunt FBARs 14. FBAR-based ladder filters are widely used in the duplexers of CDMA mobile telephones where their small physical size, large power handling capability, low cost, low insertion loss and frequency response characteristics are advantageous.
FIG. 2 is a graph showing the frequency response of FBAR-based band-pass filter 10. The frequency response exhibits a flat response in the pass band and an initial steep fall in the response in the transition between the pass band and the stop band. However, the characteristics of the band-pass filter 10 deviate from the above-described ideal characteristics because, after the initial fall, the subsequent rise in the frequency response in the stop band may result in insufficient stop band attenuation for some applications.
FIG. 3 shows an exemplary embodiment 30 of an FBAR. FBAR 30 is composed a pair of electrodes 32 and 34 and a piezoelectric element 36 between the electrodes. FIG. 3 also shows a substrate 40 that supports FBAR 30 in a way that mechanically decouples the FBAR from the substrate. Mechanically decoupling FBAR 30 from substrate 40 allows the FBAR to resonate mechanically in response to an electrical signal applied between electrodes 32 and 34. In the example shown, FBAR 30 is mechanically decoupled from substrate 40 by suspending the FBAR over a cavity 42 defined in the substrate. Other ways of mechanically decoupling the FBAR from the substrate are known.
FIG. 4 shows an exemplary embodiment 50 of a band-pass filter that incorporates a decoupled stacked bulk acoustic resonator (DSBAR) as described in above-mentioned U.S. patent application Ser. No. 10/699,289 and above-mentioned U.S. patent application Ser. No. 10/965,541. Band-pass filter 50 is based on a DSBAR 52 composed of a lower FBAR 54, an upper FBAR 56 stacked on lower FBAR 54 and an acoustic decoupler 58 between the FBARs. Each of the FBARs is similar in structure to FBAR 30 described above with reference to FIG. 3. DSBAR 52 is shown suspended over cavity 42 in substrate 40 in a manner similar to that described above. When an electrical input signal is applied between the electrodes of lower FBAR 54, upper FBAR 56 provides a band-pass filtered electrical output signal between its electrodes. The pass bandwidth of band-pass filter 50 is determined by the coupling of acoustic energy between lower FBAR 54 and upper FBAR 56. The coupling is controlled by acoustic decoupler 58. The electrical input signal may alternatively be applied between the electrodes of the upper FBAR, in which case, the electrical output signal is taken from the electrodes of the lower FBAR.
FIG. 5 is a graph showing the frequency response of DSBAR-based band-pass filter 50. The frequency response exhibits a flat response in the pass-band and a fall in the response at frequencies above and below the pass band. However, the characteristics of the DSBAR-based band-pass filter deviate from the above-described ideal characteristics in that the transition between the pass band and the stop band is less steep than some applications require. The response falls less steeply than the initial fall in the response shown in FIG. 2 of the FBAR-based ladder filter described above with reference to FIG. 1.
Accordingly, what is needed is a band-pass filter whose characteristics more closely approximate the above-described ideal characteristics. In particular, what is needed is a band-pass filter having the advantages of the above-described band-pass filters but whose frequency response in the transition between the pass band and the stop band falls more steeply than that of DSBAR-based band-pass filter 50 and that provides a greater attenuation in the stop band than FBAR-based band-pass filter 10.